In recent years, higher speed transmission, larger transmission capacity, and longer transmission distance have been demanded in optical signal transmission system. In order to meet these demands, signal processing techniques for increasing the processing speed and transmission distance of an optical signal have been sought.
As one of the optical signal processing techniques, there is a method of converting an optical signal into an electric signal, subjecting the converted electric signal to signal processing and converting the processed electric signal back into an optical signal. However, this method is unsuitable for high-speed signal processing because the method involves an extra step of converting an optical signal into an electric signal and then converting the electric signal back into an optical signal.
On the other hand, there is an all-optical signal processing technique which processes an optical signal as it is. Since this processing technique treats an optical signal directly as an optical signal without converting the optical signal into an electric signal, high-speed optical signal processing becomes possible.
In the all-optical signal processing technique, there are a method of utilizing a nonlinear optical phenomenon which occurs in an optical fiber for transmitting an optical signal, a method of utilizing a nonlinear phenomenon which occurs in an optical waveguide of material with high non-linearity, and the like.
The former all-optical signal processing technique utilizing the nonlinear optical phenomenon which occurs in an optical fiber, have been particularly attracting attentions in recent years because the technique allows high-speed processing and, at the same time, a low transmission loss. Examples of the nonlinear phenomenon which occurs in an optical fiber include four-wave mixing, self-phase modulation, cross-phase modulation, Brillouin scattering, and the like. Optical signal processing techniques such as wavelength conversion utilizing the four-wave mixing, pulse compression and waveform shaping utilizing the self-phase modulation have already been reported.
The four-wave mixing is a phenomenon in which, when two or more lights of two or more having different wavelengths propagates in an optical fiber, light of new wavelengths is generated with a specific rule by the nonlinear phenomenon. In the above-described optical signal processing techniques, this phenomenon in which light of new wavelengths is generated is utilized in wavelength conversion. In addition, this wavelength conversion utilizing the four-wave mixing has an advantage that a large number of signal wavelengths can be collectively subjected to wavelength conversion.
In addition, with shaping a waveform deteriorated during transmission by utilizing the self-phase modulation or the cross-phase modulation, all-optical signal processing, which allows a long distance transmission, is realized.
In order to actually realize the optical signal processing techniques of the wavelength conversion and the waveform shaping utilizing the nonlinear phenomena such as the four-wave mixing and the self-phase modulation in an optical fiber described above, an optical fiber which has a large nonlinear phenomena, that is, a high non-linearity is required as the optical fiber.
As the optical fiber having high non-linearity, there is one example proposed by Japanese unexamined publication No. 2002-207136 (Patent Reference 1). Characteristics at a wavelength of 1550 nm of this optical fiber are shown in detail in FIGS. 14 and 16 of the Patent Reference 1.
Dispersion Value
In the case of the optical fiber disclosed in the Patent Reference 1, a dispersion slope value at the wavelength of 1550 nm is −0.267 ps/nm2/km to +0.047 ps/nm2/km which show a large variation, and also, a dispersion value is −103.2 ps/nm/km to +3.3 ps/nm/km which show a very large absolute value in a lower limit thereof as large as 103.2 ps/nm/km.
That is, an optical fiber, which has a small variation in dispersion slope and small absolute value in dispersion at the wavelength of 1550 nm, is not provided. Therefore, an optical fiber, which has a small absolute value in dispersion over a wide wavelength region in the vicinity of the wavelength of 1550 nm, has not been provided.
Therefore, it is a first object of the present invention to provide an optical fiber which has a low dispersion value while having a high nonlinear characteristic over a wide wavelength region in the vicinity of the wavelength of 1550 nm. In addition, the present invention is to provide an optical signal processing apparatus using such an optical fiber.
Polarization Retainability Maintainability
In addition to the high nonlinear characteristic, the optical signal processing utilizing the nonlinear phenomenon is also affected by a polarization state significantly. Therefore, a polarization retaining maintaining characteristic of an optical fiber to be used is also important.
As an optical fiber which has the high non-linearity and also has the polarization retaining maintaining characteristic, there is a polarization retaining maintaining optical fiber proposed by the Patent Reference 1.
A transverse cross sectional view of this polarization retaining maintaining optical fiber is shown in FIG. 15 of the Patent Reference 1, and characteristic values thereof is shown in FIG. 16 of the Patent Reference 1.
In the case of the polarization retaining maintaining optical fiber disclosed in the Patent Reference 1, for example, positions of stress imparting members provided on opposite sides of a core, more specifically, positions of both the stress imparting members with respect to the core are not clearly indicated, and it is not clearly indicated to which degree a space interval between them should be set. Thus, there is a problem in that it is difficult to adjust polarization cross talk and a beat length in manufacturing this polarization retaining maintaining optical fiber. More specifically, there is a problem in that it is difficult to reduce the polarization cross talk to a desired allowable value.
Therefore, it is a second object of the present invention to easily manufacture and provide a polarization retaining maintaining optical fiber which has polarization cross talk of a desired small value, and is excellent in non-linearity and preferable for optical signal processing utilizing the nonlinear optical phenomenon. In addition, it is also the second object of the present invention to provide an optical wavelength converter using this polarization retaining maintaining optical fiber.
Transmission Loss
An optical fiber, which can cause the nonlinear phenomenon of a large magnitude, is obtained by increasing a nonlinear constant n2/Aeff (n2: nonlinear refractive index coefficient, Aeff: effective area) of an optical fiber. The nonlinear constant can be increased by using a material with a high nonlinear refractive index, reducing a mode field diameter of the optical fiber, or increasing a density of light to be transmitted.
A basic structure of an optical fiber comprising quartzsilica glass as a main component is constituted by a core of silica glass with a refractive index increased by germanium doping and a cladding of silica glass having a refractive index lower than that of the core, the cladding being provided on the periphery of the core.
As a quantity of germanium doped in the silica glass is increased, a nonlinear refractive index of the silica glass increases, and a refractive index thereof also increases. In addition, a mode field diameter can be reduced by increasing a difference in refractive index between the core and the cladding. Therefore, by doping a large quantity of germanium in the core, the nonlinear refractive index of the core increases, and the mode field diameter can be reduced. Thus, an optical fiber having a high nonlinear constant is obtained.
However, for obtaining an optical fiber with a high nonlinear constant, in a case where a large quantity of germanium is doped in the core to increase its non-linear refractive index and reduce the mode field diameter, a problem occurs in that a transmission loss of the optical fiber significantly increases. In general, when germanium is doped in an optical fiber, a transmission loss of the optical fiber in a wavelength band of 1550 nm increases. In particular, the increase in transmission loss of the optical fiber is remarkable, when a large quantity of germanium is doped in a core.
When the transmission loss of the optical fiber increases, even if the nonlinear constant is high, a development efficiency of the nonlinear phenomenon is deteriorated due to the large transmission loss. This can be explained by the following expressions (1) and (2).
A nonlinear phase shift ΦNL in the self-phase modulation, which is a parameter indicating non-linearity, is represented by the following expression (1).ΦNL=(2π/λ)·(n2/Aeff)·I·Leff  (1)
In the expression, n2 is a nonlinear refractive index of an optical fiber, Aeff is an effective area of the optical fiber, I is an intensity of light, and Leff is an effective length of the optical fiber.
In the above expression, n2/Aeff is a nonlinear constant.
The effective length Leff is represented by the following expression (2).Leff=[1−exp(−aL)]/a  (2)
In the expression, L is a length of the optical fiber, and a is a transmission loss of the optical fiber.
It is seen from the above expressions (1) and (2) that, when the transmission loss a of the optical fiber increases, the effective length Leff of the optical fiber decreases, and the nonlinear phase shift ΦNL also decreases.
Therefore, as an optical fiber used for the optical signal processing which utilizes the nonlinear phenomenon in the optical fiber, the low transmission loss is required as well as a high nonlinear constant. However, such an optical fiber having a high nonlinear constant and also a low transmission loss has not been found.
It is a third object of the present invention to provide an optical fiber which is devised under the above-described circumstance and has both a high nonlinear constant and a low transmission loss. 20040714
Efficiency
Moreover, in order to efficiently perform the optical signal processing which utilizes the nonlinear phenomenon, it is necessary to take into account several parameters, such as an absolute value of a wavelength dispersion value, a bending loss, a transmission loss, a length of an optical fiber, a nonlinear constant, a wavelength, and a correlation among them. It is a fourth object of the present invention to provide an optical fiber, which can efficiently perform the optical signal processing which utilizes the nonlinear phenomenon, with taking into account the correlation of these parameters.
It is a general object of the present invention to provide an optical fiber, which is appropriate for the optical signal processing utilizing the nonlinear phenomenon, and an optical signal processing apparatus which uses such an optical fiber, with taking into account the above-described several parameters comprehensively.